This invention relates to a data backup method. More specifically, this invention relates to a data backup method that uses differential snapshots.
Snapshot, which is a known method to preserve original data while maintaining consistency of the data at one point in time, is employed in computer systems for backup of data stored in storage systems and for other uses.
One type of snapshot is volume snapshot, in which a snapshot is taken by creating a copy of a primary volume which stores original data in a separate differential volume.
Another type of snapshot is differential snapshot, in which the consistency between a primary volume which stores original data and a secondary volume which stores copy data at one point in time is maintained throughout the subsequent updates to the original data to keep the original data available for reference after the updates.
According to the technique of differential snapshot, data at a time point whose consistency is to be kept intact is stored in a separate differential volume in the case where a later update to the original data is planned. In other words, with an update to the original data expected, data at a time point whose consistency is to be kept intact is stored in a separate storage volume whereas the original data is not copied without an update expected (see JP 2002-278819, for example).